Two
types of Welsh Slate feature on the new roof of London's Irish Embassy.

The
£2.5 million re-roof of the Grade II listed Irish
Embassy in London, with more than 10,000 Penrhyn Heather Blue slates from
Welsh Slate, has required its own exceptional levels of diplomacy.

DarntonB3
Architecture had multiple challenges to factor in when it came to specifying
the replacement slates, including the City of Westminster's planning
department, which was keen to see as many of the existing slates re-used as
possible and required convincing to embrace the “new” metric sizes.

Then
there were landlords Grosvenor Estates, leaseholders the Department of Foreign
Affairs and Trade, and Historic England, not to mention the site being opposite
Buckingham Palace, on a “Red route” with diplomatic bays nearby, and between
two of the most important Conservation Areas of Belgravia.

Work
began on the landlocked Grosvenor Place site in March 2018, a year after
DarntonB3, who are specialist conservation architects, were appointed to
oversee the project, and has just completed (January 2019).

Two
types of Welsh slates have been used on the former terraced town mansion with
decorative metal roof crest that was designed by architect Thomas Cundy, who
was surveyor to the Grosvenor Estate at the time, and built in 1868 in a French
Renaissance style.

Roofing
contractor Mundy Roofing was involved at an early stage in the project and were
ultimately appointed as principal contractor. Specialising in leadwork and
natural slate roofing, they were pivotal to the project's success.

A
total of 600m2 of County-grade 500mm x 300mm slates have been used
on the numerous standard 30° to 35° pitched roofs while Capital-grade bespoke arrow-head
slates of the same colour but sized at 400mm x 250mm were used around dormer
windows on the 80° mansard roof elements that are reminiscent of Paris and
feature lead secret gutter detailing around the perimeter. There is also some
vertical slating to the rear elevation.

The
old 5mm to 7mm thick slates had been on the roof since it was first built 150
years ago. The rectangular oneshad
been slightly longer and narrower, at 510mm x 255mm, while the arrow-head
slates had been smaller all round, at 350mm x 200mm.

But
a roof survey showed they were not laid to the correct bond or headlaps and
fixing points were far from ideal and non-existent in places (the headlaps
between 0 and 50mm), leading to the hardened sarking underneath becoming sodden
in places. In fact, most of the roof had been repaired over the years with
different types of slates using temporary lead tags or painted with a bitumen
solution in an attempt to prolong its life.

DarntonB3
argued that if the roof was re-installed as existing, its appearance would
alter as they would have no option but to lay the slates at the correct headlap
which would create additional courses. In addition, using the slightly larger
arrow-head slates for the mansard roof would enable them to form a more robust
detail at the abutments to the dormers and party wall.

Home
to the Irish Embassy for the past 70 years, the building comprises office and
entertainment space. The traditional timber truss roof featured timber sarking
boards with penny gaps, a form of construction usually found in Scotland. The
slates were then fixed with copper nails directly to the boarding without any
timber battens.

Once
city planners had agreed to 100% replacement of the Welsh slates, at the new
metric sizes (a process that took a year), the addition of timber counter battens,
to improve ventilation of the roof and prolong the life of the new slates, was
also proposed by the architects.

Mundy
Roofing produced sample comparison mock-ups to demonstrate to the conservation
officer this change would not be detrimental to the building's character.As it is, the interface details where slates
have been lifted has not altered the character of the building and the
introduction of battens would not be known by the general observer.

DarntonB3
senior associate Matthew Jones, who was project manager and lead architect
throughout, said: “Westminster City Council are regarded as one of the leading conservation-led
councils in the UK, with some of the highest standards and criteria to meet,
and dialogue with the conservation officer was detailed and robust. The need
for wholesale replacement of the existing slate due to them being at the end of
their life was a delicate decision and sufficient evidence of this necessity
was proven. The replacement of the slates with metric sizes was also an extensively
discussed item but the principal contractor and Welsh Slate worked with us to
develop the narrative.”

Keith
Hamilton, an architect accredited in building conservation, acting for
DarntonB3 alongside Matthew was reasonably sure Welsh slates had been used
previously but was keen to ensure the correct thickness and grading were
eventually used throughout the renewal process.

He
said “We have specified Welsh Slate on numerous other projects and their reputation
for the highest quality precedes them. We were able to argue the merits of
increased ventilation behind the slates using cross battening in lieu of direct
nailing to the existing sarking board, which in the majority of areas had
survived over 150 years' performance.

“The
risk of lack of ventilation on the lower roof pitches behind the slates was
particularly relevant at the eaves and head. We had previously considered
introducing slate vents and felt underlayto
augment any need for ventilation but the existence of the 'penny gaps' in the
sarking boards encouraged us, to believe that this was not required.

“This
was another point of continued discussion with the conservation officer who was
against an underlay in this instance. The timber sarking was found to be in
remarkable condition considering the lack of existing underlay and the direct
fix of the slates, highlighting the quality of the original slates. Hence,
there is no secondary layer apart from the slating itself and we trust the
quality of the new Welsh slate will replicate the existing quality and last
another 100 years.”

Due
to tight access on the roof, they were also able to widen the lead gutters and
set back the lower courses of slates to avoid getting them broken. All the new Penrhyn
slates were holed and traditionally fixed with 38mm copper nails as opposed to
clipped or other methods. The standard-size slates were able to cope with the
wide variation of roof pitches and new rooflights encountered by varying the
lap and gauge slightly.

In
virtually every case, the slate junctions are with lead or copper flashings and
as it was appreciated there is some risk of staining, all lead was treated with
patination oil. Using new treated timber battens for fixing the new slates proved
a great success as direct fixing into the old hardened sarking boarding would
have been a major problem and time consuming.

Matthew
said the support they had received from Welsh Slate had been “fantastic” and
included a site visit to match the type of slate, a letter explaining the need
to change from imperial to metric sizing, technical drawings of the arrow-head
slates, technical information on the end life of slates and their unsuitability
for re-use, on-time deliveries with little if no wastage, and recommending
experienced slating contractors.

Mundy
Roofing were on site for a total of 10 months. Work included the rebuilding of
three chimney stacks, involving 30 tonnes of stone and brickwork, and
restoration of traditionally-forged wrought ironwork to the pavilion roof
crest,all underneath a temporary roof.

Russell
Mundy said: “This project was extremely challenging due to it being a working
embassy but Welsh Slate were excellent with their support in achieving planning
consent and the product has received widespread praise from the client.”

Matthew
said: “Due to the extremely difficult access to this roof, we were conscious to
use a slate that will require little maintenance (if any) and satisfy
appearance for an extended period of time. Welsh Slate were able to provide
technical studies comparing different types of slate and their longevity. This
enabled the landlord, Grosvenor Estates, that the new roof should outlast the
previous roof and match it entirely with other buildings nearby. Welsh Slate's
evidence on the existing slates being at the end of their useful life, and the
lifecycle information of the new slates, helped give the conservation officer
comfort that the right approach was being taken for the building.”

Keith
added: “Essentially, Welsh Slate ensured we got the right slate, quality,
consistency and sizes for the varying roof pitches and conditions. There have
been no problems with mixing batches or colour variations which can occur. They
also met the stringent programme requirements, reducing risks of delay from the
main contractor. To our knowledge, there have been no rejects on quality of
slates.

“The
final result is the new slating looks exactly like it was envisaged in 1868,
except with the knowledge it is better-fixed and easier to reach for
maintenance, with a discreet fall arrest system fitted. The client is extremely
happy with the quality of the final works.”

Andrea
Fox, senior architect with the property management unit of the Department of
Foreign Affairs and Trade, said: “The team have provided us with an exceptional
and beautiful project that we know will stand the test of time and one we are
extremely proud to have commissioned.”

“I
want to thank the Welsh Slate team for the support they provided during our
project and especially in relation to protracted issues relating to the listed
building consent approval.”